a) Field of the Invention
The invention is directed to a control system for a scanner drive, especially for a laser scanning microscope, with an oscillating motor for driving an oscillating mirror serving for the linearly oscillating deflection of a beam bundle, with a control unit for supplying the oscillating motor with exciting current which is variable with respect to control frequency, frequency curve, and amplitude, with a function generator which is connected with the control unit, and with a measurement value transducer for obtaining a sequence of information about the deflection positions of the oscillating mirror.
b) Description of the Related Art
Optical devices with scanning arrangements, including laser scanning microscopes, are known in principle in the art. A laser which focusses light along a beam path onto a small light point, generally called a pixel, in a focal plane is typically used as a radiation source. In this way, virtually all of the laser light is guided to this individual target point.
The scanning device of an instrument of the kind mentioned above serves for the linear deflection of the light coming from the laser as well as the light reflected from the object plane and, in this respect, for moving the light point in the image plane or in the object plane. A raster scanning device which is controlled synchronously with the scanner emits the resulting detector output signal as image information.
For oscillating deflection of the beam path, it is known to provide electromechanically driven mirrors and to deflect the beam path in such a way that the target point moves in the direction of an axis which will be called the x-axis. For this purpose, the mirror can direct the laser bundle onto a second mirror which is driven in the same way and which causes a movement of the target point in the direction of an orthogonal axis, the y-axis.
The deflection in the x-axis will be considered more closely in the following. Although the deflecting mirrors that are used have smaller dimensions and accordingly have less mass, the problem in such scanning devices consists in always generating fast and accurate mirror movements for the purpose of good image linearity with short image formation times. This is because the mirror movement or beam path follows the drive signals emitted by the control unit with only varying degrees of faithfulness due to different interference influences. This is not adequate for a highly efficient scanning device in which the demand for high scanning frequency must always be met and in which it is required that the target point maintains a constant speed over the entire deflection phase.
In order to obtain drive characteristics for the deflecting mirror which are as linear as possible, a control signal with a triangular wave is generated in the control unit. The phases and amplitude of a drive signal of this kind form the basic precondition for approximation of the deflection to linear movement of the target point depending on time.
It is known in the art to use harmonic analysis, i.e., the determination of Fourier coefficients, for the purpose of the resultant approximation of a triangular wave. A scanning device of this kind with associated control unit is described, e.g., in DE-OS 4322694. In this case, control signals are generated on the basis of two of the Fourier components, giving a relatively good resultant approximation of a triangular wave. The type of control shown in this case disadvantageously leads to unsatisfactory results because the two frequencies are treated differently by the scanner according to amplitude and phase. This is the case even when additional harmonics of the fundamental frequency are used for additional correction. In other words, the solution suggested in this case is not suitable for realizing the desired linearization.
In the publication mentioned above, two resonant scanners and a galvanometer scanner are provided for deflection of the laser beam in the x-axis, wherein the galvanometer scanner is used to superpose a DC oscillating movement on the resonant movement supplied by the resonant scanners. As is well known, the oscillating movement of a resonant scanner is caused to a great extent by the exchange of energy between the motion of a mass, especially the mirror, and the deflection of an elastic element, such as a spring, to which the mass is attached.
In a departure from the construction described thus far according to which a plurality of separate scanners are operated within the scanning device, each with its own resonant frequency, it is known to operate an individual scanner with a plurality of resonant frequencies. For example, U.S. Pat. No. 4,859,846 describes the operation of a scanner which works with a mirror and generates a plurality of resonant frequencies for this scanner by means of a suitable control system. This system is also a resonant scanner system. This solution is also unsuitable for overcoming the disadvantage that the actual deflection position is falsified by the position predetermined by the control signal because of various interfering influences, e.g., external temperature influences, influencing variables associated with material, etc.